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Severe storms are localised events, usually affecting smaller areas than tropical cyclones and floods, so their devastating impact is often under-estimated. These storms, which are more common than any other natural hazard, can occur anywhere in Australia. Each year, on average, severe storms are responsible for more damage (as measured by insurance costs) than tropical cyclones, earthquakes, floods or bushfires. Unfortunately, storms also kill people: between 5 and 10 deaths are caused by lightning strikes each year. More deaths occur when strong winds cause tree limbs to fall, debris to become projectiles and small boats in open water to capsize. Although many people believe that tornadoes do not occur in Australia, they have caused at least 41 deaths here.

Severe storms: definitions, causes and occurrence

A severe thunderstorm is defined by the Bureau of Meteorology as one which produces; hail, diameter of 2 cm or more ($2 coin size); or wind gusts of 90 km/h or greater; or flash floods; or tornadoes, or any combination of these.

What causes severe thunderstorms

Thunderstorms require 3 main ingredients; a source of moist air, an unstable atmosphere and a trigger mechanism. Moist air is important because when it condenses to form cloud, heat energy is released making the rising air more buoyant and fuelling further cloud growth. An unstable atmosphere is necessary so that developing cloud is able to rise freely to great heights in the atmosphere.

Trigger mechanisms are important as they serve as a focus for storm development. Typical triggers are cold fronts, heat troughs and regions of low pressure. Hills and mountains may also enhance storm development. The severity of any subsequent thunderstorms will depend largely on the buoyancy of the rising air within the storm and the structure of the wind within the atmosphere. Wind direction and speed is rarely constant, and generally tends to increase in speed and turn anti-clockwise (in the southern hemisphere) with increasing altitude. The change in wind direction and speed as you move upwards through the atmosphere is known as "wind shear".

Certain instability and wind profiles can lead to the development of long-lived thunderstorms with strong rotation within their cores. These storms are known as "Supercells" and are responsible for the majority of damage caused by severe thunderstorms. Supercell thunderstorms may produce very large hail, extraordinarily strong wind gusts, powerful tornadoes and heavy rainfall.

Lightning and thunder

Lightning is the discharge produced when voltage differences between ground and atmospheric electrical charge are large enough (several hundred million volts) to overcome the insulating effect of the air. Strokes can occur within the cloud, between clouds, or between clouds and the ground. Thunder is the sound produced by the explosive expansion of air heated by the lightning stroke to temperatures as high as 30,000°C.

Hail

Hailstones can form in a thunderstorm with a strong updraught when frozen raindrops, suspended in the updraught, grow rapidly by sweeping up small cloud droplets which freeze on contact. Hailstones larger than cricket balls have been recorded in Australia.

Wind gusts

In a mature thunderstorm, the falling rain and hail drag the surrounding air downwards. In addition, evaporation from the raindrops cools the nearby air, accelerating the downward rush. This strong downdraught spreads out upon reaching the ground, producing a cool, gusty wind that can cause damage.

The Canberra Times has covered the official launch of ACT First, reporting that, "Too many Canberra residents are failing to take simple steps to safeguard their families and properties against storms and floods."

The Canberra region has experienced an average of one earthquake per year for the past 50 years; and for most residents this information would come as a surprise. Frequency of occurrence of earthquakes within 20 km of Canberra GPO Seventy-two earthquakes occurred within 20 km of the GPO in the 36 years prior to the end of 1995, and during this period yearly numbers varied between zero and nine with a mean number of 2.0.

Flash floods can occur almost anywhere there is a relatively short intense burst of rainfall such as during a thunderstorm. As a result of these events the drainage system has insufficient capacity or time to cope with the downpour. Although flash floods are generally localised, they pose a significant threat because of their unpredictability and normally short duration.

105 people have been killed by landslide events since 1842 in 100 recorded landslide events (National Landslide Database, 2007). Many of these events resulted from natural phenomenon, though half of those causing death and injury can be attributed to human activity.

Green Cross Australia brought a whole new twist to disaster preparedness with a zombie flashmob in Canberra. Zombie-clad locals invaded Canberra’s Old Bus Depot Markets, performing to Michael Jackson’s Thriller – all to support ACT First. See photos of the event on the Canberra Times...

Severe heatwaves were once expected to hit Canberra about once every 25 years, experts say, but the sweltering conditions will be the third such hot spell in the capital in four years.
The Canberra Times reports the blistering heat is a sign of things to come, with the CSIRO predicting ACT residents could suffer through 26 days a year with a top temperature of 35 degrees or more before the end of the century.

A community of ACT social media users will add to front-line preparations this year to prepare Canberrans for their next natural disaster. Read John Thistelton's report from the Canberra Times on the launch of ACT First.

Storms can happen anywhere, but they are more common in the ACT from September to end of February. Severe storms are more common than any other natural hazard and are responsible for more damages measured by insurance costs than bushfires, floods or cyclones.

During a ‘heatwave’ temperatures are above average for several sequential days and it remains hot overnight. Heatwaves have caused more deaths than bushfires in Australia, and cause losses to crops, livestock and infrastructure.

Many people believe that tornados do not occur in Australia; this is not true, they do and have caused numerous deaths. Most thunderstorms do not reach the level of intensity needed to produce these dangerous phenomena, but they all produce lightning which can cause death, injury and damage. Tornados can and do occur in the ACT.